In communication systems, the duration of call setup (or call setup delay) is a main factor affecting Quality Of Service (QOS). For services sensitive to such delays, such as interactive games and Push to talk Over Cellular services (PoC), the call setup delay in current UMTS systems (usually 6 to 10 seconds) appear to be too long.
The following description is a brief introduction of a call setup process in a typical UMTS system, using a User End (UE) calling another UE as an example. The call setup process flow is illustrated in FIG. 1 and FIG. 2. These figures incorporate several main functional units, such as a User End (UE), a NodeB, a Radio Network Controller (RNC), and a Core Network (CN). In addition, both figures presume that a user initiates a PoC service call in a Packet Service (PS) domain, and a Radio Resource Control (RRC) connection is established on a Dedicated Channel (DCH).
For the originating UE, a call setup process usually includes the following steps:                1) RRC connection establishment        2) Non Access Stratum (NAS) signaling connection establishment and NAS signaling exchange        3) Radio Access Bearer (RAB) establishment        
For the receiving UE, the call setup process is similar to that of the originating UE and mainly includes the following steps:                1) Paging process        2) RRC connection establishment        3) Non Access Stratum (NAS) signaling connection establishment and NAS signaling exchange        4) Radio Access Bearer (RAB) establishment        
The following description further analyzes each step of the process described above. The purpose of establishing a RRC connection is to build a dedicated signaling connection between a UE and a UTRAN (UMTS Terrestrial Radio Access Network including several RNCs and NodeBs) for transferring signaling between the UE and the UTRAN, or between the UE and the CN. In FIG. 1, phase 1:RRC depicts a RRC connection establishment procedure when the UE is in an idle mode.
The purpose of establishing the NAS signaling connection is to build a signaling connection between the UE and the CN. The NAS signaling connection establishment and signaling exchange are shown as Phase 2:NAS in FIG. 1 and FIG. 2. The signaling mainly include Service Request for the originating UE, IU connection (between the RNC and CN), Signaling Connection Control Part (SCCP) establishment, authentication and encryption request, Security Mode configuration, Temporary Mobile Subscriber Identity (TMSI) reallocation, Activate Packet Data Protocol (PDP) activation request, etc.
After NAS signaling connection is established, RAB establishment is started as shown by phase 3:RAB in FIG. 1 and FIG. 2. The CN sends a RAB Assignment Request to the RNC. After receiving the request, the RNC maps from the RAB to a Radio Bearer (RB), and then reconfigures RL (Radio Link), establishes ALCAP, and synchronizes FP to NodeB. Then, the RNC sends a RB Setup message to the UE. After receiving this message, the UE configures RLC, MAC and physical layer and then sends a RB Setup Complete message to the RNC. The RNC, after configuring RLC and MAC, sends a RAB Assignment Response to the CN to notify the successful setup of the RAB at the UTRAN side. The CN then sends an Activate PDP Context Accept message to the UE to inform the UE that data transfer can now commence. Thus, RAB setup is now successful.
From the analysis above, it is clear that the call setup process involves many signaling/data connection processes that result in a long call setup delay. The call setup delay in a UMTS system is longer than that of a GSM system. Therefore, for services sensitive to call setup delay, it is necessary to reduce the call setup delay in a UMTS system in order to improve user satisfaction.